1. Field of the Invention
The present invention relates to a pixel structure, and more particularly, to a pixel structure of a Thin Film Transistor (TFT) array substrate and a repairing method thereof.
2. Description of the Prior Art
The Thin Film Transistor-Liquid Crystal Display (TFT-LCD) is the most popular Flat Panel Display (FPD) recently. It has many benefits such as its low power consumption, thin shape, light weight, and low driving voltage, etc.
It is known that the TFT-LCD has been developed towards to the application field of television in recent years, so the display panel has been promoted to the large-scale design. Consequently, the fabrication process is becoming more and more complex and difficult. It is hard to consider the influence of both the constraint conditions and the process errors on the display quality at the same time although there are other key factors to affect the productivity and the yield.
The display region of a TFT array substrate contains a plurality of pixels arranged in matrix. A pixel is defined by the crossing of two scan lines and two data lines. In addition to a pixel electrode, there are a TFT element and a storage capacitor contained in the pixel. The TFT is a switching element and its on-off state is controlled by both gate signals from the scan line and source signals from the data line. A storage capacitance line is provided in the pixel to form a capacitor that can maintain the present pixel signal of the pixel electrode until the subsequent signal is applied thereto.
During the fabrication process of the TFT array, it is liable to be damaged and resulted in an abnormal short-circuit or open-circuit by several factors such as the static electricity and the unexpected particle pollution. The dot defect of the pixel can be distinguished as several kinds such as the white defect and the black defect, etc. In order to assure the display quality, normally the black-picture inspection and the white-picture inspection will be executed to find the dot defect of the pixel after the completion of the substrate for the TFT array and the color filter. Since the white defect is always bright in the black inspection, which human eyes are very sensitive to recognize. Generally, a laser repairing process will be adopted when just a few white defects are found in the black-picture inspection.
A partial top-view schematic diagram of a pixel with the conventional laser repairing structure in the TFT array substrate is illustrated in FIG. 1A. A Data line 114 transmits a source signal to a source electrode 100, wherein the data line 114, the source electrode 100 and a drain electrode 106 are located in the second conductive layer. A scan line 104 transmits a gate signal, wherein the scan line 104 is located in the first conductive layer on a substrate. A storage capacitance line 110 formed with the first conductive layer in the pixel is provided to transmit the common voltage (Vcom). The semiconductor electrode 102 is partially covered by the source electrode 100 and partially covered by the drain electrode 106. A contact hole 108 is used to electrically connect the pixel electrode 112 and the drain electrode 106.
Once the pixel is found to be a white defect in the black-picture inspection, the laser beam is used to irradiate the overlapped region 118 of the drain electrode 106 and the scan line 104 so as to electrically connect the drain electrode 106 and the scan line 104. Because the pixel electrode 112 and the drain electrode 106 are electrically connected, the electrical voltage of the pixel electrode 112 is equal to the electrical voltage of the gate electrode after the laser repair.
The electrical voltage of the gate electrode is alternated between the high-level voltage (Vgh) and the low-level voltage (Vgl). Further, absolute values of the voltage difference between Vgh, Vgl and Vcom are bigger than the Vcom so as the repaired pixel will always display as a dark defect.
For example, FIG. 1B is to illustrate the electrical voltage of the repaired pixel. Vgh, Vgl and Vcom are respectively equal to 24V, −6V, and 4V. The absolute value of the voltage difference between Vgh and Vcom, and Vgl and Vcom are respectively Vd1 and Vd2. Vd1 and Vd2 are respectively equal to 20V and 10V. Thus, both Vd1 and Vd2 are bigger than Vcom. Therefore, the repaired pixel will display as a dark defect so as to achieve the repair purpose for the white defect.
This kind of laser repairing structure and method for repairing a white defect eliminates the drawback of being bright for a pixel permanently. Nevertheless, it will display as an obvious dark point in the white-picture inspection so as to degrade the display quality of the TFT-LCD panel.
FIG. 2A is another partial top-view schematic diagram of a pixel with the conventional laser repairing structure in the TFT array substrate. A Data line 214 transmits a source signal to a source electrode 200, wherein the data line 214, the source electrode 200 and a drain electrode 206 are located in the second conductive layer. A scan line 204 transmits a gate signal, wherein the scan line 204 is located in the first conductive layer on a substrate A storage capacitance line 210 formed with the first conductive layer in the pixel is provided to transmit the common voltage (Vcom). The semiconductor electrode 202 is partially covered by the source electrode 200 and partially covered by the drain electrode 206. A contact hole 208 is used to electrically connect the pixel electrode 212 and the drain electrode 206. One floating metal conductor 216 formed with the first conductive layer is prepared for the laser repair in necessity. The floating metal conductor 216 is respectively partially overlapped with the data line 214 and the drain electrode 206 at the overlapped regions 218 and 220.
Once the pixel is found to be a white defect in the black-picture inspection, the laser beams can be used to irradiate the overlapped regions 218 and 220 from the lower surface side of the substrate to electrically connect the data line 214 with the floating metal 216 and to electrically connect the drain electrode 206 with the floating metal 216. Thus, the data line 214 and the drain electrode 206 are electrically connected through the floating metal 216. Then, the source signal can directly transmit to the pixel electrode 212 through the contact hole 208 to convert the white defect into a gray defect so as to achieve the repair purpose for the white defect.
However, the gray defect will flick owing to the alternation of the positive-negative polarities in both the black-picture inspection and the white-picture inspection.
The electrical voltages of the repaired pixel are illustrated in FIG. 2B and FIG. 2C. FIG. 2B is to illustrate the alternation of the positive-negative polarities in the black-picture inspection. The source signal is alternated between the high-level voltage (Vsh) and the low-level voltage (Vsl). The Vsh, Vsl and Vcom are respectively equal to 8V, 0V and 4V. Voltage differences between Vsh and Vcom, and Vsl and Vcom are respectively Vd3 and Vd4. Vd3 and Vd4 are respectively equal to 4V and −4V. Thus, the gray point will flick owing to the alternation of the positive-negative polarities.
FIG. 2C is to illustrate the alternation of the positive-negative polarities in the white-picture inspection. The source signal is alternated between the high-level voltage (Vsh) and the low-level voltage (Vsl). Vsh, Vsl and Vcom are respectively equal to 5V, 3V and 4V. Voltage differences between Vsh and Vcom, and Vsl and Vcom are respectively Vd5 and Vd6. Vd5 and Vd6 are respectively equal to 1V and −1V. Thus, the gray point will flick owing to the alternation between the positive-negative polarities.
Therefore, this kind of laser repairing structure and method for repairing white defect is free of the drawback of being always bright or always dark for a pixel. However, it will make the pixel flick when the positive-negative polarity of the source signal is changing, and so as to degrade the display quality of the TFT-LCD panel.